The MI6 spy found murdered in his home was a quiet and modest man who never broke espionage etiquette by talking about his job, his friends revealed yesterday.

Gareth Williams, 31, gave nothing away about himself and no one saw him come and go from his top floor flat.

But the avid cyclist’s stealthy lifestyle gives detectives few clues as to why he was brutally targeted by his attacker.

His decomposed remains lay undiscovered in a large bag in his bathtub for up to two weeks after he was killed. He is thought to have been strangled.

Officers are investigating whether his gruesome death could be linked to his top secret codebreaker’s job. They are also looking at his personal life as he is thought to have let his attacker into the five-storey house.

mystery

Dismissing suggestions of an assassination by foreign agents as fantasy yesterday, security sources said it was possible he may have been attacked by a friend or casual acquaintance.

They have also not ruled out the possibility another intelligence officer could be involved and plan to interview MI6 staff and Mr Williams’ colleagues.

Yesterday officers were quizzing sister Ceri in Chester, where neighbours recalled Mr Williams as “the perfect spy”. Ex-police officer Cathy Stanley said: “He was quiet, kept to himself and gave nothing away about himself. Classic spy profile, I suppose.” His uncle William Hughes added: “He worked for GCHQ for years. He’d never talk about his work and it felt rude to ask.”

Devastated former landlady Jenny Elliott, who rented him a flat above her garage in Cheltenham when he worked at GCHQ, said he was “like a mouse”.

The 71-year-old added: “I came to regard him almost like a son. I used to say ‘Oh go on, tell me what you do’ but I didn’t badger him because they weren’t allowed to say. He would just give me a cheeky chuckle and one of his beautiful smiles which lit up the room.” Police broke into the flat in Pimlico, Central London, after colleagues grew concerned when he did not show up for work.

Mr Williams had been living there for a year during a secondment to MI6 from the Government’s listening post in Cheltenham. He was due to return to GCHQ next week.

A postmortem failed to establish how Mr Williams was killed but it is thought he was strangled or suffocated.

Detectives found his mobile phone and several SIM cards strewn around the floor. The £1million house is owned by a company registered in British Virgin Islands.

But insiders said it was routine practice for the UK intelligence services to use foreign firms to purchase accommodation for its staff. It is understood at least three previous residents have links to GCHQ and other agencies.

Neighbours had no idea Mr Williams was a spy and were shocked when police sealed off the street on Monday.

Secretary Laura Houghton, 30, said: “His curtains were often closed. I could never tell if anyone was in. We never saw him come and go.”

GENIUS

Other neighbours include former home secretaries Michael Howard and Lord Brittan.

Mr Williams excelled at maths and was talent-spotted at Cambridge University. He cycled up to 200 miles a week competing in races for a club in Cheltenham.

Devastated parents Ia n and Ellen returned from holiday abroad to help formally identify their son.

It is the first murder on British soil of someone linked to secret services since Alexander Litvinenko in 2006. He drank radioactive polonium-210 in tea.

A GCHQ spokesman said: “We do not comment on individual members of staff or whether they are staff.”

* Did you know Gareth Williams or do you have any photos of him. Ring the newsdesk free on 0800 282 591.

LISTENING IN ON WORLDShrouded in secrecy, GCHQ is one of the world's largest eavesdropping operations.

Its futuristic circular base, nicknamed The Doughnut, employs 5,500 in Cheltenham, Glos, to intercept millions of phone calls, texts, emails and coded messages from around the world.

Analysts like maths graduate Gareth Williams, decipher the messages, passing on clues to MI6, MI5 and Scotland Yard, which could pinpoint a Taliban commander, uncover a plot against Britain, or help a drugs bust.

Known to staff simply as HQ, it was founded after World War One and in World War Two broke the crucial German Enigma submarine code at Bletchley Park, Bucks.

Cops have to solve it quickBy Crispin BlackJames Bond spends much of his time avenging fellow spooks killed by the bad guys.

But back in the real world it has always been rare for British spies to die of anything other than natural causes.

Neverthless, the murder of Gareth Williams will have sent shudders through the intelligence establishment. Senior officers from all three intelligence services will be urgently reviewing the facts assisted by policemen from Scotland Yard, specially vetted to allow them access to secret intelligence.

There are two main possibilities. The first is that the murder was connected in some way to the victim's intelligence work - the Russians are up to their old tricks or a terrorist organisation has somehow been able to target GCHQ personnel.

More likely is that the murder was connected to Mr Williams's life. He was murdered by someone he knew. This after all is what happens to most victims. There is a chance he may have been a victim of random violence. Whatever happened, the authorities need to get to the bottom of the matter fast. Given that Mr Williams had a PhD in Applied Mathematics from Cambridge University it is clear that he was a codebreaker. His body remained undiscovered possibly for a fortnight - we need to be sure that national security has not been compromised.